18: We Are Stardust

For the past two episodes, we’ve been learning about the origins of life of Earth. We haven’t met anything living yet, but that’s OK- we’re starting from scratch. In Episode 16, we started at the beginning and met the element carbon, the fundamental element in life as we know it. Pure carbon can be found in ancient minerals such as diamonds or graphite. Carbon also likes to mix with other elements- hydrogen, oxygen, and nitrogen -to make more complicated structures: molecules.

Last episode, we learned about three delicious carbon-rich molecules that are key ingredients for life on Earth and for our diets: proteins, carbohydrates, and fats. These molecules are not alive, but they give your body the material and energy it needs to grow. In short, before we can make living things, we first need to make their building blocks.

So, how did complex molecules like proteins, carbs, and fats form in the first place?

Today, we tackle that very question and return back to Earth’s distant past. This episode doesn’t have a specific date in the Earth Calendar, but we’re floating sometime in the Hadean Chapter between January and February, before 4.0 billion years ago. These are Earth’s missing years, with nothing remaining except a few lonely zircon crystals. We know Earth had liquid water at the time, but direct evidence of life is still inconclusive- check out Episode 16 for a refresher.

Without direct evidence, we need other windows into the hidden Hadean world to learn how life was first built. Scientists have several tricks up their sleeves, each with pros and cons. In short, we can look for the building blocks of life in three places: 1) in space, 2) in the natural world, or 3) in lab experiments. Today, we’ll focus on the search in space, but first we need to add one more word to our vocabulary, to make our lives a bit easier. This is a word which means one thing in the grocery store, but something very different in chemistry. The word of the day is “organic”.

Part 1: An Organic Buffet

Let’s consider the molecules we met last episode: proteins, carbs, and fats, as well as their smaller cousins, sugars and amino acids. These pieces have many differences, like their sizes, shapes, and their roles in the human body. And yet, chemists lump all of these materials together under one large tent. They are all called “organic molecules”. So what does that mean exactly?

Does “organic” mean molecules were made with free-range food with no antibiotics, pesticides or preservatives? No- this is the “organic” definition used by farmers and governments to classify food.

Does “organic” mean these molecules are only found in living things? No, but it’s not a bad question. In biology and environmental science, “organic matter” is any material that used to be part of a living thing. But remember, there was once a time when nothing was living in the universe.

In chemistry, the word “organic” means something far more simple.

So what makes organic molecules special? Their atomic ingredients. If you want to be an organic molecule, there are only two rules to follow.

Rule #1: Organic molecules must have carbon and at least one other element.

Rule #2: There must be at least one link between two carbon atoms or carbon and hydrogen.

And that’s it! Let’s line up a few everyday items and see if they pass the test.

First, table salt. Salt only has the elements sodium and chlorine. No carbon, so it’s not organic.

Next, the diamond in a wedding ring. The diamond is pure carbon, but it fails Rule 1- there are no other ingredients besides carbon. The same holds true for graphite in your pencils.

How about the carbon dioxide we breathe out, or CO2? CO2 has one carbon atom and two oxygen atoms, so it passes Rule 1. However, that single carbon isn’t attached to another carbon or hydrogen, so CO2 isn’t organic.

Now let’s look at a simple carbohydrate like sugar. Sugar instantly passes the organic test: sugar has many carbon atoms attached to hydrogen, as do all other carbo-hydrates- it’s right in the name. Proteins and fats have completely different shapes, but they have lots of interlinking carbon and hydrogen atoms, so they too are organic molecules.

DNA, a complex organic molecule

As you can imagine, with only two simple rules to become an organic molecule, many other things on Earth also fit the bill. We would be here all day if I listed every example, but some other organic families include vitamins, alcohol, methane, rubber, petroleum, plastics, and DNA. If you’re in your house or at work, take a second and see if you can identify some organic molecules around you. There are probably more than you realize, especially if you have objects made from living things.

And yet, on their own, none of these items are alive. A pile of rubber, a pool of oil, a strand of hair or a slice of meat is not the same as a living organism. Even DNA, the code of life inside every cell, is not itself alive. A lonely pile of DNA would just sit and rot over time. And yet, just a few of these materials combined in different ways make the building blocks of every living thing on Earth today.

Some of these molecules are extremely simple: methane has just five atoms. On the other hand, DNA has billions of atoms braided together in a curved ladder. A general rule going forward is this: the more complex a molecule is, the more materials and energy you need to make it, and the harder it is to form. Making methane from scratch is easy, making DNA from scratch is very tricky. Our new friends proteins, carbs, and fats, are somewhere in the middle- difficult, but not impossible to make. All you need are the right conditions.

Going forward, we will see organic molecules forming in many places from seabeds to geyser fields. But to begin, let’s return to our old stomping grounds from earlier this season- the space between worlds.

Part 2: The Smell of Outer Space

If there’s one thing to remember from the Hadean, it’s that everything on Earth had its’ origins in outer space. We’ve seen asteroids clump together to make our planet, we’ve seen the Moon’s birth from a titanic planetary collision, and icy asteroids crash into Earth and and melt into a global ocean in Episode 14.

Organic molecules can also be made in space- it’s a fact that astronauts have learned by following their noses.

Let’s pretend that you’re an astronaut on the International Space Station, doing a routine spacewalk. You’re tethered safely to the spaceship, but it’s still a bit unnerving. Below you is the Earth, our homeworld laid out on a scale where titanic mountains appear like tiny ridges. On the other side is a sea of stars on an even larger, incomprehensible scale, with infinite leagues of cold dark space in between.

It’s intimidating, but don’t worry, I’m not going to put you through any traumatic events. It’s time to return into the safety of the space station. You enter the airlock, which closes and vents in fresh air. Taking off your helmet, you take a deep relaxing breath through your nose, and smell… burnt steak. The odor quickly clears as you make your way into the space station- it wasn’t a cooking mishap, it was something in the airlock with you and it lingers on your spacesuit… but what’s causing that smell?

This strange experience has been reported by many astronauts, with different odors including hot metal, gunpowder, ozone, barbeque, and burnt almond cookies. The broad notes seem to be something burning or metallic. There are several ideas to explain the smell, and most have to do with tiny molecules floating in space, hitchhikers from the void.

Way back in Episode 4, we learned that space is not quite so empty as it appears. There are atoms and molecules floating around, just far fewer than in the air we breathe. The distances between most atoms in space is like two beach balls floating on either end of the Pacific Ocean. This very sparse mixture is called the Interstellar Medium, which I still think would be a great name for a Space Wizard.

Most of the Interstellar Medium is made of hydrogen and helium, but the closer we look, we do find pockets of more complicated material, including organic compounds. Now we could test this around Earth by smell alone, but how do we know what’s in the Interstellar Medium around the universe, in areas millions of light-years away? We haven’t sent anyone that far yet just to sniff the air.

The answer is hidden in starlight.

Whenever light hits an object, some is absorbed in and some is reflected away. That reflected or rejected light, travels to our eyes and gives the world color. For example, a red apple absorbs every color of light except red. A blueberry reflects blue light. A blackberry absorbs or takes in all colors of light, while a white egg reflects and sends all colors back to the eyes.

Something similar happens with tiny atoms and molecules in space. Let’s take a hydrogen atom for example. Most colors of light bounce right off hydrogen, just like an egg. But it does absorb specific shades of red and blue. Every atom and every molecule has a special color-coded fingerprint- some orange, green or purple, or usually a big messy mix. But again, we can’t travel to the middle of the Milky Way to see the colors of every atom. 

Fortunately, there’s a much easier way to tell. Instead of looking for individual molecules, we look at the starlight that’s bounced off. If you have a glass prism or piece of crystal jewelry, now’s a great time to take it out. If not, just whip out Pink Floyd’s Dark Side of the Moon album cover. When light hits a prism or piece of crystal, it smears apart into a rainbow, separating reds from blues.

Absorption spectrum of several atoms

Astronomers can do the same thing with light traveling through space- they’re literally turning starlight into rainbows for science. Pretty cool, right? It gets even cooler. Let’s say that starlight has passed through a bunch of hydrogen atoms. The hydrogen will gobble up certain shades of red and blue, literally removing these colors from starlight. When we split the light into a rainbow, there are literally chunks missing, a series of thin black lines just like a barcode at a grocery store. In the same way, different atoms and molecules have different barcodes, different series of chopped up rainbows.

This technique is called spectroscopy, since a rainbow can also be called a spectrum. In fact, light is just one part of a much wider spectrum of energy, including radio, microwaves, ultraviolet and X-rays. All these waves interact with molecules in different ways, giving us many tools to see the universe beyond our own limited eyes or noses.

With that in mind, let’s return to space and meet the building blocks of life between the stars.

Part 3: Return to the Cradle of Stardust

The interstellar medium is mostly hydrogen and helium, only 0.1% is anything else. But that’s the 0.1% we’re interested in today. In Episode 4, we saw tiny grains of crystal and iron floating in the void, the bricks that would build our planet. If we turn our spectroscopes to the sky and peer through shattered rainbows and radio waves, we see other molecules as well.

One example of a Polycyclic Aromatic Hydrocarbon (PAH)

One common character only has two atoms: carbon and hydrogen, it’s the simplest organic molecule: CH. There’s no CH in your body, it’s very unstable on Earth but it manages just fine in outer space. In fact, CH was the first space molecule discovered by spectroscopy in 1937. Over the decades, hundreds of more complex characters have been discovered. Most are organic with lots of carbon and hydrogen, especially larger molecules with ten or more atoms.

Of these, there is a special family, one we will follow for the next few episodes. They have larger, complex structures and longer, complicated names. These are polycyclic aromatic hydrocarbons, or PAHs for short. I wouldn’t throw that name at you if I didn’t think it was important. Everything we need to know about this molecule is in its name. Let’s work backwards. H is for hydrocarbon, so it’s an organic molecule. A is for aromatic- in chemistry, an aromatic molecule is shaped like a ring, or a donut. Finally, P is for polycyclic- poly means “many”, and cycle means circle or ring.

So PAHs are organic molecules shaped like many rings. In short, PAHs look like brass knuckles, chicken wire, or chain mail. The A for aromatic also solves a mystery from earlier this episode. In normal life, aromatic is a fancy word for “smelly”. The reason space smells like burning is thanks to our new friends the PAHs, which are also made whenever you burn wood or meat.

So we’ve met tiny molecules like CH and large, smelly ones like PAHs, but neither of them are in living things, so why do we care? These two relatives are probably the ancestors for the proteins, carbs, and fats in your body and your diet. To see how, we need to return to an old location from Episode 4, a nebula.

Orion Nebula, cradle of stars, planets, and organic matter

As a reminder, a nebula is a cloud of concentrated Interstellar Medium- a city of atoms in the vast emptiness of space. Nebulae are where stars and planets are born, but today we’ll focus on a far smaller scale.

To make a new organic molecule in a nebula, you either need to shove together smaller molecules like CH or split apart larger ones like PAHs. Scientists are still working out the details, but there are two main recipes- shaking and baking. I’m not an astrophysicist, so please forgive me for glossing over the details.

One way to shove molecules together is by pure force, which is provided by exploding stars. As they blow up, shock waves scream through the universe like ripples on a pond. These waves force atoms closer to each other, where they’re more likely to interact. Another way is through starlight itself, especially ultraviolet or UV light. UV light has a lot of energy, and likes to break things apart. This is why too much UV light is bad for your skin and you start to sunburn. In space, UV light can mess around with atoms, giving them an electric charge. Even in a cold distant nebula, opposites attract, and charged atoms can assemble together into new, larger forms. On the other hand, UV can also act as a wrecking ball, dissolving larger molecules like the smelly PAHs.

In short, while space is extremely empty, nebulae provide playgrounds where atoms can make and break bonds to form new molecules. If we turn our spectroscopic eye into a nebula, we start to see some familiar faces from our ham sandwich last episode- amino acids, the building blocks of proteins, primitive carbohydrates, and very recently, a key building block of fats. We haven’t made life yet, but we’re finally starting to assemble the pieces together. The next step is bringing them down to the Hadean Earth.

Summary: Organic molecules are very diverse, from natural substances like wood and protein to human-made objects like plastic or gasoline. They all share the same thing- bonds between carbon and hydrogen. These bonds were first forged between the stars, in clouds mixed by interstellar shock waves and UV radiation. From this simple recipe, we can see that space is filled with the building blocks of life, even if we haven’t found life itself. But we’ll get there. To quote Carl Sagan and Joni Mitchell, “We are stardust, we are billion-year-old carbon.”

Next episode, we hitch a ride back to Earth, to see how our world took these basic ingredients and made the first living things.

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